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In the present study, we report on the synthesis of nanocrystalline phase pure ZnAl2O4 by a simple citrate solegel method. The materials annealed at different temperatures have been characterized for their structural and optical properties. The near infrared reflective properties of the samples are emphasized for use as solar reflective materials.
Journal of Science: Advanced Materials and Devices (2019) 524e530 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Nanostructured zinc aluminates: A promising material for cool roof coating S Sameera, Viji Vidyadharan, Sajesh Sasidharan, K.G Gopchandran* Dept of Optoelectronics, University of Kerala, Thiruvananthapuram, Kerala 695581, India a r t i c l e i n f o a b s t r a c t Article history: Received 15 May 2019 Received in revised form 10 October 2019 Accepted 20 October 2019 Available online 28 October 2019 Nanostructured zinc aluminate pigments were synthesized using a citrate solegel method All the samples were annealed at 600 C, 700 C, 800 C, and 1100 C for h, respectively The as-synthesized samples were characterized by X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), field emission scanning electron microscopy (FE-SEM), UVeViseNIR spectroscopy and CIE L*a*b* color analysis The phase purity and structural analysis of the zinc aluminates were confirmed using X-ray diffraction and Rietveld refinement The morphological and microchemical analysis was done using FESEM, TEM and EDX The particle size and reflectance properties are found to be sensitive to the annealing temperature The samples exhibited a total solar reflectance of about 85% An attempt was also made to compare the total solar reflectance of the synthesized zinc aluminates with that of commercially available TiO2 by applying over a concrete cement surface Crown Copyright © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/ 4.0/) Keywords: Zinc aluminate Nanocrystalline materials Citrate-sol gel method NIR reflectance Cool coating Introduction Solar reflective pigments have been a keen study of interest in recent years, for their applications as thermal-cooling materials in the current scenario of worldwide warming Solar reflective coatings are characterized by their ability to reflect visible and near infrared parts of the sunlight, thus reducing the heat absorbed by buildings, automotives and pavements, leading to energy savings [1] Therefore, a more comfortable interior climate and electricity savings with air conditioning during the hot season can be achieved A substantial portion of the energy expenditure adds up from the construction sector Also, buildings are responsible for nearly 70% of sulfur oxide emissions, 50% of carbon dioxide emissions [2] and a large proportion of smog [3] Some recent research reports have estimated a possible saving of peak cooling load of 11e27% for air-conditioned buildings depending on the climatic conditions [4] Hence, materials with high solar reflective properties offer an effective passive cooling method for reducing an urban heat island effect and greenhouse gas emissions [5,6] * Corresponding author E-mail address: gopchandran@yahoo.com (K.G Gopchandran) Peer review under responsibility of Vietnam National University, Hanoi Among the solar reflective materials, rutile TiO2 - a white pigment is currently regarded as the best pigment for coating materials, but it is also the most expensive Mixed metal oxidebased materials in nanoscale have been a subject of interest due to their fascinating optical, structural, magnetic, catalytic and thermal properties Zinc aluminate (ZnAl2O4) is a member of the spinel oxides having the general formula AB2O4, where A represents a divalent metal ion and B represents a trivalent metal ion This is a widely sought material possessing extremely diversified properties such as high thermal and mechanical resistance, hydrophobic nature, high fluorescence efficiency, high chemical stability, high quantum yields and low sintering temperature [7e9] ZnAl2O4 is a direct band gap semiconductor with an optical band gap of around 3.8 eV [10] Thus, ZnAl2O4 spinel is a nontoxic, lowcost material and has high thermal stability Various chromophores such as cobalt, chromium, manganese have been introduced into the spinel lattice to produce colored pigments [11e14] Various methods of synthesis have been employed for the synthesis of ZnAl2O4 The shortcomings of a conventional solidstate reaction method, such as inhomogeneity, lack of stoichiometry control, high temperature and low surface area, are ameliorated when ZnAl2O4 is synthesized using a solution-based method Nanomaterials based on ZnAl2O4 have been prepared by various soft chemical routes such as coprecipitation [15], hydrothermal [16], glycothermal [17], solvothermal [18], combustion [19], https://doi.org/10.1016/j.jsamd.2019.10.003 2468-2179/Crown Copyright © 2019 Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 polymeric precursors [20], pyrolysis [21], solegel [8], mechanical alloying [22] methods and explored for their various properties And the aid of surfactants and other inorganic materials utilized in these preparation procedures is costly which limits its industrial applicability The motivation of this work is to synthesize highquality ZnAl2O4 nanomaterial with impressive solar reflectance compared to the widely used titania as a possible alternative nontoxic white pigment for reducing the heat build-up Among the various synthesis methods, the solegel method ensures an easy mixing of precursor solutions at the molecular level and provides a high degree of homogeneity at relatively low processing temperatures It enables a higher uniform particle size distribution and non-agglomeration [23] Citric acid is commonly used in the solegel method, the advantages being readily available and cheap Also, citric acid is an effective chelating agent The citric acid solegel or citrate solegel method is normally used for the synthesis of binary, ternary and quaternary metal oxides in both crystalline and amorphous forms The conversion of the ‘gel’ to a mixed metal oxide is accomplished by pyrolysis in air, with the maximum temperature depending on the specific system As the metalecitrate ‘gels’ are heated, the organic component undergoes combustion at around 300e400 C, depending on the metal counterion and the presence of additives Another impact of the homogeneity of citrate solegel precursors is on the reaction temperature since the final crystalline metal oxide may be formed at significantly lower temperatures than those using powder solid-state methods [24] In the present study, we report on the synthesis of nanocrystalline phase pure ZnAl2O4 by a simple citrate solegel method The materials annealed at different temperatures have been characterized for their structural and optical properties The near infrared reflective properties of the samples are emphasized for use as solar reflective materials Also, we have investigated the solar reflective property of the as-prepared ZnAl2O4 over a concrete surface Experimental 2.1 Synthesis of ZnAl2O4 nanoparticles ZnAl2O4 nanoparticles were prepared via a citrate solegel method, in which zinc nitrate hexahydrate [Zn(NO3)2$6H2O] (98%, Sigma Aldrich), aluminium nitrate nonahydrate [Al(NO3)3$9H2O] (Emplura, Merck) and citric acid (99.5%, Sigma Aldrich) were taken as the starting materials At first, stoichiometric amounts of Zn(NO3)2$6H2O and Al(NO3)3$9H2O were dissolved in distilled water by ultrasonication The citrate solution was prepared by dissolving the appropriate amount of citric acid in dilute nitric acid and both solutions were mixed The citric acid to metal molar ratio was fixed as 2:1 The mixed solution was heated at 90 C in a water bath for h and then magnetically stirred for h The obtained solution was kept at room temperature until a gel was formed which was dried at 200 C for h After grinding the dried sample, it was annealed separately at 600 C, 700 C, 800 C and 1100 C for h with a heating rate of 10 C/min The obtained powders were ground in an agate mortar and used for further characterization 525 Spectrometer in the range 400e4000 cmÀ1 using Quest single reflection ATR accessory and KBr window The resolution was cmÀ1 with 40 accumulation scans at a speed of 0.2 cm/s The field emission scanning electron microscope (FE-SEM) images were obtained by means of a field emission scanning electron microscope FEI Nova nano SEM 450 operating at an acceleration voltage of 15 KV The energy dispersive X-ray analysis (EDX) of the samples was carried out using Carl Zeiss EVO 18 secondary electron microscope with an EDAX attachment of AMETEK EDAX octane series attached to the SEM using the software TEAM The high-resolution micrographs were obtained from JEOL JEM 2100 transmission electron microscope (TEM) using a 200 kV electron beam The ultravioletevisibleenear-infrared (UVeViseNIR) spectra were measured by UVeVisibleeNIR spectrophotometer (PerkinElmer, LAMBDA 950 with an integrating sphere attachment) using spectralon as a standard The color coordinates were evaluated by coupling color analytical software UV WinLab to the spectrophotometer The color of the samples was evaluated according to The Commission Internationale del' Eclairage (CIE) through L*a*b* color scales The measurement conditions were as follows: an illuminant D65 and a 10 complementary observer In this system, L* is the color lightness (L* is zero for black and 100 for white), a* is the green (À)/red (ỵ) axis, and b* is the blue ()/yellow (ỵ) axis The total solar reflectance is expressed as the integral of the percent reflectance times the solar irradiance divided by the integral of the solar irradiance when integrated over the 300e2500 nm range as shown in the formula, ð 2500 R¼ rðlÞiðlÞdl 300 ð 2500 300 (1) iðlÞdl where r(l) is the spectral reflectance obtained from the experiment and i(l) is the standard solar spectrum (WmÀ2 mmÀ1) The NIR solar reflectance spectra were determined from ASTM Standard G173-03 [25] 2.3 Application studies To explore the potential of the above pigments for cool roof applications, the NIR reflectance study of selected pigments was conducted on a concrete cement slab The preparation method of the coating on the concrete cement slab as a cool coat is as follows At first, the pigment (10 wt.%) was ultrasonicated for 10 to ensure the complete dispersion in polyurethane (PU diol solution, Sigma Aldrich) which acted as the binder The resulting viscous solution was coated on a cm  cm  cm concrete cement slab and subsequently dried in air to obtain a coating For comparison, the above process was repeated with rutile TiO2 nanopowder (99.5%, Sigma Aldrich) Results and discussion 2.2 Characterization 3.1 Crystal structure and morphology The crystalline nature and phase purity of the synthesized samples were characterized by X-ray powder diffraction (XRD) using a powder X-ray diffractometer (Bruker D8 ADVANCE) with Ni-filtered Cu Ka (l ¼ 1.5405 Å) radiation Data were collected by step scanning over a 2q range from 10 to 80 with a step size of 0.02 Rietveld analysis of the XRD data was carried out using Topas program (Version 4.1) Fourier transform infrared spectroscopy (FTIR) data were measured using PerkinElmer Frontier FIR XRD patterns of the annealed samples are shown in Fig 1(a) The XRD data confirms that the samples are phase pure in nature which can be indexed to the cubic spinel ZnAl2O4 corresponding to the powder diffraction file number 05-0669 of ICDD database No extra peaks related to any other impurity was detected The crystallite size was calculated from Debye Scherrer formula D ¼ 0.9l/bcosq (2) 526 S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 Fig (a) Powder XRD patterns of the zinc aluminates annealed at various temperatures and (b) An expanded view of the (311) peak at q around 36 for the same samples where D is the crystallite size, l is the wavelength of X-ray used, b and q are the half-width of X-ray diffraction lines and half diffraction angle of 2q The diffraction peak of maximum intensity (3 1) was used to determine the crystallite size The crystallite sizes varied from 13 to 23 nm for the samples annealed from 600 C to 1100 C Rietveld refinement of the XRD patterns was employed for the estimation of structural parameters using the TOPAS program The XRD powder patterns were simulated employing the space group Fd3m with Zn at 8a, Al at 16d and O at 32e Wyckoff positions respectively A fundamental parameter approach was used for fitting the diffraction peaks The refined parameters are listed in Table The lattice constant for the nanostructured zinc aluminates obtained here is in good agreement with the reported value (a ¼ 8.0848 Å) of ZnAl2O4 Fig 1(b) shows the expanded view of the (311) peak at 2q around 36 It is observed from Fig 1(b) that the reflection slightly shifts to higher angles indicating a contraction of lattice for samples annealed up to 800 C The graphical output of the Rietveld refinement of ZnAl2O4 samples is shown in Fig S1 It is seen that the best possible fits are obtained in all the samples FT-IR spectra of the samples are shown in Fig For the sample ZnAl2O4 annealed at 600 C, a small band at 3471 cmÀ1 is seen which corresponds to OH group indicating slight water content in the sample The vibrations corresponding to the spinel structure are well observed around 652, 548 and 475 cmÀ1 The spinels exhibit stretching bands in the 500e900 cmÀ1 range [21], which are associated with the vibrations of metal-oxygen, aluminiumoxygen and metal-oxygen-aluminium [26] These bands correspond to the regular spinel structure with six-fold coordinated aluminium [8] No other phases are identifiable in the FT-IR spectra which is consistent with the XRD results FE-SEM micrographs presented in Fig reveal that the particles are homogeneous and almost spherical in shape The average sizes of the particles are found to be in the range of 25e35 nm Fig 4(a) illustrates the transmission electron microscopic analysis (HR-TEM) of representative ZnAl2O4 sample annealed at 1100 C Particle size distribution is in the range of 25e30 nm calculated from HR-TEM images through ImageJ software It can be seen that agglomeration observed in the FE-SEM is also detected in TEM In Fig 4(b), the HRTEM clearly shows that the spacing between periodic fringes is 0.24 nm which is in agreement with the crystal plane space for (1 1) plane of ZnAl2O4 and is in agreement with the XRD analysis The corresponding selected area electron diffraction (SAED) pattern shown in Fig 4(c) indicates the nanocrystalline nature of the sample Hence, the TEM, XRD and SEM results are in agreement with each other 3.2 Chemical analysis Elemental analysis using (EDX) revealed that the obtained stoichiometric composition is very close to the theoretical composition in the analyzed regions The peaks at 2.1e2.2 keV in Table Structural parameters of ZnAl2O4 samples annealed at different temperatures Sample 600 C Lattice parameters a (Å) 8.1161(8) V(Å)3 534.61 R-factors Rexp (%) 9.14 Rp (%) 6.87 Rwp (%) 9.58 RBragg (%) 9.64 GOF 1.05 700 C 800 C 1100 C 8.0951(1) 530.49 8.0098(1) 513.89 8.0742(2) 526.39 9.70 7.36 10.18 16.56 1.05 9.54 7.10 10.15 8.88 1.06 9.25 6.87 9.92 13.56 1.07 The fixed atomic fractional coordinate positions are Zn [8a] (0.125, 0.125, 0.125), Al [16d] (0.5, 0.5, 0.5) and O [32e] (0.264, 0.264, 0.264), in space group Fd3m Fig FT-IR spectra of the zinc aluminate samples annealed at different temeratures S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 527 Fig FE-SEM micrographs of the zinc aluminate samples annealed at different temperatures Fig (a) TEM (b) HR-TEM images and (c) SAED pattern of of ZnAl2O4 annealed at 1100 C the EDX spectra are due to the gold, which is coated on the samples before analysis Thus, the formation of pure ZnAl2O4 is confirmed by EDX analysis which corroborates to the XRD analysis Fig S2 shows the EDX spectra of all the samples 3.3 UVevisible studies and heat reflection performance The UVevisible absorption spectra of the ZnAl2O4 samples are shown in Fig Here the Kubelka Munk reemission function which is used as a measurement of absorption via powder, to convert reflectance spectrum to the absorption spectrum [27] An absorbance peak around 275 nm is observed for all the samples This is due to the fundamental band-to-band electron excitations (electron transition between filled O 2p orbitals and empty Al 3s orbitals, with the possibility of mixing of 3s and 3p wavefunctions for Al3ỵ) and related to the band structures of intrinsic properties of spinels A shoulder in the range of 330e400 nm is observed on the main peak due to the electronic excitation between filled O 2p and empty Zn 4s orbitals which indicates some defects [28] This Fig Absorbance spectra of the zinc aluminate samples annealed at different temperatures (Inset: Colorimetric parameters) 528 S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 ahn ¼ A(hnÀEg)n Fig NIR reflectance spectra of ZnAl2O4 samples annealed at various temperatures shoulder disappears in the case of ZnAl2O4 sample annealed at 1100 C indicating the absence of defects A change in the absorption edge of the ZnAl2O4 samples is seen as the annealing temperature increases ZnAl2O4 is considered to be a direct band gap semiconductor [29] The band gap energy (Eg) of the ZnAl2O4 samples can be determined from plots of (ahn)2 versus hn using the Tauc relation [30], where h is the Planck constant, a is the KubelkaeMunk (KeM) absorption coefficient, and n is the frequency (3) The plots of (ahn)2 versus hn for all the samples are shown in Fig S3 The extrapolation of linear regions of the plots give the direct band gap values of 4.07, 3.83, 3.73 and 3.78 eV respectively which is close to that of reported literature [10] The inset of Fig shows the CIE L*a*b* color coordinates of the samples with comparison to rutile TiO2 Near-infrared irradiation (NIR) lying in the 700e2500 nm accounts for 52% of the energy in the solar irradiance spectrum [31] The reflectance of the samples increases with annealing temperatures in the infrared wavelength due to the increasing roughness of the surface of the grains The NIR reflectance spectra of ZnAl2O4 samples annealed at various temperatures are shown in Fig Some absorption is also seen in the 1350e2500 nm region which may arise from combination and overtones of fundamental processes that occur in the mid-infrared region Since nanocrystalline oxides have a high surface area, more water molecules, carbonate ions, etc will be adsorbed and hence exhibit strong absorption features So the reflectance in 700e1300 nm region is taken into account here Also, the particular wavelength at 810 nm is selected because many photonic devices operate around this wavelength [32] The highest NIR reflectance of 86% is achieved in ZnAl2O4 annealed at 1100 C (Table 2) Fig shows the NIR solar reflectance spectra of the ZnAl2O4 samples determined by ASTM standard G173-03 As mentioned earlier, in the entire solar spectrum, 52% is composed of NIR (700e2500 nm), which can be further carved up into the shortwave NIR (700e1100 nm) and longwave NIR Table Reflectance values of ZnAl2O4 samples annealed at different temperatures ZnAl2O4 600 C 700 C 800 C 1100 C Average reflectance in Vis region (%) Average reflectance in 700e1300 nm region (%) NIR reflectance at 810 nm (%) Total solar reflectance (%) 75 83 83 78 72 82 81 76 78 81 80 78 85 87 86 85 Fig NIR solar reflectance spectra of ZnAl2O4 samples annealed at various temperatures S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 (1100e2500 nm) [33] Among them, shortwave NIR is the main heat-generating area The NIR solar reflectance spectra displayed in Fig shows the sunlight radiation energy distribution of prepared samples It can be seen that the radiant energy distribution is mainly located in the shortwave NIR range (700e1100 nm) with a distinct decrease occurring only at 1500e2500 nm However, the prepared ZnAl2O4 samples exhibit a total solar reflectance above 75% This is imputable to the fact that total solar reflectance covers the entire range of radiation between UV and NIR These results summarize that the ZnAl2O4 samples have the potential to reduce heat build-up 3.4 Application studies over concrete surface To assess the utility of the nanopigments for energy saving applications, their applicability was checked on a building roofing material like concrete cement The particular ZnAl2O4 nanopigment exhibiting the highest NIR reflectance annealed at 1100 C was selected to prepare cool coating Single-layer cool white coatings are more suitable for cooling of reinforced concrete roofs, while multilayer ones may be better for cooling of metal-based roofs [34] The most commonly used pigment in the manufacturing of cool coatings is titanium dioxide (TiO2) Of the available white pigments, rutile TiO2 is known to exhibit the strongest near-infrared light reflection [35] The NIR reflectance spectra of the resulting coatings coated over bare concrete cement surfaces are shown in Fig A bare concrete 529 surface exhibits a low NIR reflectance of 27%, while the bare concrete coated with ZnAl2O4 and TiO2 have NIR reflectances of 90% and 87% in the 700e2500 nm region, respectively The total solar reflectance and CIE color coordinates were measured in concrete cement as described earlier The CIE color coordinates of ZnAl2O4 coated concrete cement substrates are (L* ¼ 97.98, a* ¼ À2.34, b* ¼ 7.52) Fig shows the NIR solar reflectance spectra of bare concrete coated with ZnAl2O4 and TiO2 The photographs of the resulting coated samples are shown in the inset of Fig The total solar reflectance of ZnAl2O4 coated concrete is 89% while that of TiO2 coated concrete is 87% The results point out that a cool roof coating based on ZnAl2O4 nanopigment can enhance the NIR reflectance, which leads to a reduction in the surface temperature of the roof Conclusion The nanostructured ZnAl2O4 pigments have been prepared by the citrate solegel method The XRD results confirm the formation of a single-phase cubic ZnAl2O4 structure Rietveld analysis was performed to determine the structural parameters As the annealing temperature increased, the crystallinity of the samples increased, resulting in a slight increase in grain size Remarkable NIR reflectance of 83e87% was observed in the 700e1300 nm range in the nanostructured ZnAl2O4 pigments The developed pigments could confer their NIR reflecting properties to the concrete substrate under study Since these nanopigments act as cool coatings with high total solar reflectance, they may serve as potential energy-saving materials Declaration of Competing Interest The authors declare no conflict of interest Acknowledgments S.Sameera acknowledges the financial support from the University Grants Commission (UGC), Govt of India, New Delhi towards Dr D S Kothari Postdoctoral fellowship program (Ref No F.4-2/2006 (BSR)/CH/16-17/0043) Viji Vidyadharan acknowledges to SERB, DST (India) for National PDF (PDF/2016/002564/PMS) The authors thank Soumya Valsalam from Central Laboratory for Instrumentation and Facilitation (CLIF), University of Kerala, for the EDX analysis Fig NIR reflectance spectra of ZnAl2O4 and TiO2 coated over concrete surfaces Appendix A Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jsamd.2019.10.003 References Fig NIR solar reflectance spectra of ZnAl2O4 and TiO2 coated over concrete surfaces (Inset: Photographs of the coatings) [1] H Akbari, M Pomerantz, H Taha, Cool surfaces and shade trees to reduce energy use and improve air quality in urban areas, Sol Energy 70 (3) (2001) 295e310, https://doi.org/10.1016/S0038-092X(00)00089-X [2] F Hammad, B Abu-Hijleh, The energy savings potential of using dynamic external louvers in an office building, Energ Buildings 42 (2010) 1888e1895, https://doi.org/10.1016/j.enbuild.2010.05.024 [3] S.E Bretz, H Akbari, A Rosenfeld, Practical issues for using solar-reflective materials to mitigate urban heat islands, Atmos Environ 32 (1998) 95e101, https://doi.org/10.1016/S1352-2310(97)00182-9 [4] A Synnefa, M Santamouris, H Akbari, Estimating the effect of using cool coatings on energy loads and thermal comfort in residential buildings in various climatic conditions, Energ Buildings 39 (11) (2007) 1167e1174, https://doi.org/10.1016/j.enbuild.2007.01.004 [5] M Santamouris, A Synnefa, T Karlessi, Using advanced cool materials in the urban built environment to mitigate heat islands and improve thermal comfort conditions, Sol Energy 85 (12) (2011) 3085e3102, https://doi.org/ 10.1016/j.solener.2010.12.023 530 S Sameera et al / Journal of Science: Advanced Materials and Devices (2019) 524e530 [6] H Akbari, H.D Matthews, Global cooling updates: reflective roofs and pavements, Energ Buildings 55 (2012) 2e6, https://doi.org/10.1016/ j.enbuild.2012.02.055 [7] M Zawadzki, Synthesis of nanosized and microporous zinc aluminate spinel by microwave assisted hydrothermal method, Solid State Sci (1) (2006) 14e18, https://doi.org/10.1016/j.solidstatesciences.2005.08.006 [8] F Davar, M Salavati-Niasari, Synthesis and characterization of spinel-type zinc aluminate nanoparticles by a modified solegel method using new precursor, J Alloys Compd 509 (5) (2011) 2487e2492, https://doi.org/10.1016/ j.jallcom.2010.11.058 [9] L Cornu, M Duttine, M Gaudon, V Jubera, Luminescence switch of Mn-doped ZnAl2O4 powder with temperature, J Mater Chem C (44) (2014) 9512e9522, https://doi.org/10.1039/C4TC01425A [10] S.K Sampath, D.G Kanhere, R Pandey, Electronic structure of spinel oxides: zinc aluminate and zinc gallate, J Phys Condens Matter 11 (18) (1999) 3635e3644, https://doi.org/10.1088/0953-8984/11/18/301 [11] A.A Ali, E El Fadaly, I.S Ahmed, Near-infrared reflecting blue inorganic nanopigment based on cobalt aluminate spinel via combustion synthesis method, Dyes Pigments 158 (2018) 451e462, https://doi.org/10.1016/j.dyepig.2018.05.058 [12] A Ferna0 ndez-Osorio, E Pineda-Villanueva, J Ch avez-Fern andez, Synthesis of nanosized (Zn1-xCox)Al2O4 spinels: new pink ceramic pigments, Mater Res Bull 47 (2012) 445e452, https://doi.org/10.1016/j.materresbull.2011.10.024 [13] H.R Hedayati, A.A Sabbagh Alvani, H Sameie, R Salimi, S Moosakhani, F Tabatabaee, A Amiri Zarandi, Synthesis and characterization of Co1-xZnxCr2yAlyO4 as a near-infrared reflective color tunable nano-pigment, Dyes Pigments 113 (2015) 588e595, https://doi.org/10.1016/j.dyepig.2014.09.030 [14] R Yang, A Han, M Ye, X Chen, L Yuan, The influence of Mn/N-codoping on the thermal performance of ZnAl2O4 as high near-infrared reflective inorganic pigment, J Alloys Compd 696 (2017) 1329e1341, https://doi.org/10.1016/ j.jallcom.2016.12.100 [15] S Farhadi, S Panahandehjoo, Spinel-type zinc aluminate (ZnAl2O4) nanoparticles prepared by the co-precipitation method: a novel, green and recyclable heterogeneous catalyst for the acetylation of amines, alcohols and phenols under solvent-free conditions, Appl Catal A Gen 382 (2) (2010) 293e302, https://doi.org/10.1016/j.apcata.2010.05.005 [16] C.C Yang, S.Y Chen, S.Y Cheng, Synthesis and physical characteristics of ZnAl2O4 nanocrystalline and ZnAl2O4/Eu coreeshell structure via hydrothermal route, Powder Technol 148 (2004) 3e6, https://doi.org/10.1016/ j.powtec.2004.09.011 [17] W Walerczyk, M Zawadzki, H Grabowska, Glycothermal synthesis and catalytic properties of nanosized Zn1-xCoxAl2O4 (x ¼ 0, 0.5, 1.0) spinels in phenol methylation, Catal Lett 141 (2011) 592e601, https://doi.org/10.1007/ s10562-011-0546-x [18] W Staszak, M Zawadzki, J Okal, Solvothermal synthesis and characterization of nanosized zinc aluminate spinel used in iso-butane combustion, J Alloys Compd 492 (2010) 500e507, https://doi.org/10.1016/j.jallcom.2009.11.151 [19] R Ianos, R Lazau, I Lazau, C Pacurariu, Chemical oxidation of residual carbon from ZnAl2O4 powders prepared by combustion synthesis, J Eur Ceram Soc 32 (2012) 1605e1611, https://doi.org/10.1016/j.jeurceramsoc.2011.12.028 [20] L Gama, M.A Ribeiro, B.S Barros, R.H.A Kiminami, I.T Weber, A.C.F.M Costa, Synthesis and characterization of the NiAl2O4, CoAl2O4 and ZnAl2O4 spinels by the polymeric precursors method, J Alloys Compd 483 (2009) 453e455, https://doi.org/10.1016/j.jallcom.2008.08.111 [21] D Dhak, P Pramanik, Particle size comparison of soft-chemically prepared transition metal (Co,Ni,Cu,Zn) aluminate spinels, J Am Ceram Soc 89 (2006) 1014e1021, https://doi.org/10.1111/j.1551-2916.2005.00769.x [22] G Maity, P Maji, S Sain, S Das, T Kar, S.K Pradhan, Microstructure, optical and electrical characterizations of nanocrystalline ZnAl2O4 spinel synthesized by mechanical alloying: effect of sintering on microstructure and properties, Physica E 108 (2019) 411e420, https://doi.org/10.1016/ j.physe.2018.10.024 [23] D.T.T Nguyet, N.P Duong, T Satoh, L.N Anh, T.T Loan, T.D Hien, Crystallization and magnetic characterizations of DyIG and HoIG nanopowders fabricated using citrate sol-gel, J Sci Adv Mater Devices (2016) 193e199, https://doi.org/10.1016/j.jsamd.2016.05.005 [24] A.E Danks, a S.R Hallb, Z Schnepp, The evolution of ‘solegel’ chemistry as a technique for materials synthesis, Mater Horiz (2016) 91e112, https:// doi.org/10.1039/C5MH00260E [25] ASTM G 173-03, Standard Tables for Reference Solar Spectral Irradiances: Direct Normal and Hemispherical on 37 Tilted Surface, ASTM International, West Conshohocken, PA, 2012 [26] J Perez-Ramirez, G Mul, F Kapteijn, J.A Moulijn, In situ investigation of the thermal decomposition of CoeAl hydrotalcite in different atmospheres, J Mater Chem 11 (3) (2001) 821e830, https://doi.org/10.1039/B009320N [27] C Ding, A Han, M Ye, Y Zhang, L Yao, J Yang, Hydrothermal synthesis and characterization of novel yellow pigments based on V5ỵ doped BiPO4 with high near-infrared reectance, RSc Adv (2018) 19690e19700, https:// doi.org/10.1039/C8RA02406E [28] S.K Sampath, James F Cordaro, Optical properties of zinc aluminate, zinc gallate and zinc aluminogallate spinels, J Am Ceram Soc 81 (2007) 649e654, https://doi.org/10.1111/j.1151-2916.1998.tb02385.x [29] H Dixit, N Tandon, S Cottenier, First-principles study of possible shallow donors in ZnAl2O4 spinel, Phys Rev B 87 (2013) 174101, https://doi.org/ 10.1103/PhysRevB.87.174101 [30] C Ragupathi, J Judith Vijaya, A Manikandan, J.L Kennedy, Phytosynthesis of nanoscale ZnAl2O4 by using sesamum (Sesamum indicum L.) optical and catalytic properties, J Nanosci Nanotechnol 13 (2013) 8298e8306, https:// doi.org/10.1166/jnn.2013.7922 [31] A Synnefa, M Santamouris, K L Apostolaki, On the development, optical properties and thermal performance of cool colored coatings for the urban environment, Sol Energy 81 (2007) 488e497, https://doi.org/10.1016/ j.solener.2006.08.005 [32] P Jeevanandam, R.S Mulukutla, M Phillips, S Chaudhuri, L.E Erickson, K.J Klabunde, Near infrared reflectance properties of metal oxide nanoparticles, J Phys Chem C 111 (2007) 1912e1918, https://doi.org/10.1021/ jp066363o [33] C Ding, A Han, M Ye, Y Zhang, L Yao, J Yang, Synthesis and characterization of a series of new green solar heat-reflective pigments: Cr-doped BiPO4 and its effect on the aging resistance of PMMA (Poly(methyl methacrylate)), Sol Energy Mater Sol Cells 191 (2019) 427e436, https://doi.org/10.1016/ j.solmat.2018.12.007 [34] Z Song, W Zhang, Y Shi, J Song, J Qu, J Qin, T Zhang, Y Li, H Zhang, R Zhang, Optical properties across the solar spectrum and indoor thermal performance of cool white coatings for building energy efficiency, Energ Buildings 63 (2013) 49e58, https://doi.org/10.1016/j.enbuild.2 013.03.051 [35] R Levinson, P Berdahl, H Akbari, Solar spectral optical properties of pigmentsdpart II: survey of common colorants, Sol Energy Mater Sol Cells 89 (2005) 351e389, https://doi.org/10.1016/j.solmat.2004.11.013 ... https://doi.org/10.1016/j.materresbull.2011.10.024 [13] H.R Hedayati, A. A Sabbagh Alvani, H Sameie, R Salimi, S Moosakhani, F Tabatabaee, A Amiri Zarandi, Synthesis and characterization of Co1-xZnxCr2yAlyO4 as a near-infrared... respectively A fundamental parameter approach was used for fitting the diffraction peaks The refined parameters are listed in Table The lattice constant for the nanostructured zinc aluminates obtained... Science: Advanced Materials and Devices (2019) 524e530 Fig (a) Powder XRD patterns of the zinc aluminates annealed at various temperatures and (b) An expanded view of the (311) peak at q around 36 for